Remove PathLocator::getStageDirectory()

Problem/Motivation

In #3250696: Each stage should have its own staging directory, to avoid filesystem and locking conflicts, we're making every stage have a unique staging directory path. Therefore, PathLocator::getStageDirectory() is kind of useless, since its assumption is that there's one overall staging directory. Let's just remove it and rely on each stage telling us what its staging directory should be.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes